1. Field of the Invention
The present invention relates to a stereoscopic apparatus and method for producing images that can be displayed as three-dimensional illusions and more particularly to an autostereoscopic imaging apparatus and method for producing images that, on display, can be perceived to be three-dimensional without the use of special display devices or viewing aids.
2. Background Information
The production of two-dimensional images that can be displayed to provide a three-dimensional illusion has been a long standing goal in the visual arts field. Methods and apparatuses for producing such three-dimensional illusions have to some extent paralleled the increased understanding of the physiology of human depth perception.
Binocular (stereo) vision requires two eyes to view a scene with overlapping visual fields. Each eye views a scene from a slightly different parallax angle and focuses the scene onto a retina. The two-dimensional retinal images are transmitted by optic nerves to the brain's visual cortex, where they are combined, in a process known as stereopsis, to form a three-dimensional model of the scene.
Depth perception of three-dimensional space depends on various kinds of information (cues) perceived from the scene being viewed, such as relative size, linear perspective, interposition, light and shadow, gradients (monocular cues), as well as retinal image size, retinal disparity, accommodation, convergence (binocular cues), and familiarity with the subject matter of the viewed scene (learned cues).
True three-dimensional image displays can be divided into two main categories, stereoscopic or binocular and autostereoscopic. Stereoscopic techniques (including stereoscopes, polarization, anaglyphic, Pulfrich, and shuttering technologies) require the viewer to use a viewing device, such as polarized glasses. Autostereoscopic techniques do not. Some autostereoscopic techniques, such as holography, lenlicular screens and parallax barriers require special display devices. Others, such as alternating-pairs and parallax scans require neither special viewing devices nor special display devices.
For example, prior art autostereoscopic television and motion picture systems have utilized the approach of alternately displaying views of a scene recorded by two cameras from different points of view using a conventional display device, i.e., a display device not uniquely designed to provide or assist in providing a three-dimensional illustration. U.S. Pat. No. 4,006,291 to Imsand; U.S. Pat. Nos. 4,303,316 to McElveen; U.S. Pat. No. 4,429,328 to Jones et al.; U.S. Pat. No. 4,966,436 to Mayhew & Pritchard, all utilized two cameras to record horizontally, vertically, or a combination of horizontally and vertically displaced views of a scene. While this autostereoscopic approach produces images which provide three-dimensional illusion when displayed with a conventional display device, precision matching of the two cameras is required. Improper alignment of the cameras, lens mismatches in focal length and/or focus, chrominance and illuminance mismatches, and misplaced convergent points, all contribute to image instability. Also, considerable operator skill is required to continuously adjust disparity and convergence of the two cameras in a coordinated manner to maintain a stable image.
Image instability can be rendered less noticeable by the use of masking techniques. Camera motion is very effective in hiding rocking motions of images, apparently because the brain places less importance on rocking motion than on camera motion. This could result from some sort of natural stabilizing phenomena or mechanism of the brain that allows us to see clearly while walking or running, when images would otherwise bounce.
To avoid the drawbacks associated with a two-camera autostereoscopic system, autostereoscopic methods and apparatuses using a single camera/single lens have been developed. Mayhew et al. U.S. Pat. Nos. 5,014,126 and 5,157,484 disclose single camera autostereoscopic systems capable of recording images which, when displayed on a conventional device, are perceived by a viewer in three-dimension. Commonly assigned, copending U.S. patent application Ser. No. 08/115,101, filed Sep. 2, 1993 by Fernekes et al. discloses a method and apparatus, wherein a single camera records images while undergoing parallax scanning motion. The disclosures of the Mayhew et al. patents and the Fernekes et al. application are incorporated herein by reference.
As more fully described in these references, the optical axis of a single camera is made to move in a repetitive pattern which causes the camera optical axis to be offset from a nominal stationary axis. This offset produces parallax information. Motion alone is such a pattern is referred to as parallax scanning motion. As the motion repeats the pattern, the motion, in effect, becomes oscillatory, with each completion of the pattern being considered a scan cycle.
The pattern may be repeated with each cycle, or many change. In either event, there is a need to coordinate the rate each cycle is repeated by the camera (scanning cycle rate) and the rate that images recorded during each cycle are displayed (view cycle rate). The view cycle rate is the rate of display of the parallax information produced during each scan cycle and, to achieve optimum results, should fall in the range of 3 to 6 view cycles per second, although satisfactory results might be achieved at a rate as low as 2 view cycles per second, or as high as 15 view cycles per second.
In the case of a single camera moving in a vertical plane from a top point to a bottom point of view, the resulting parallax scanning motion may be considered as following the circumference of a circle as seen from the circle's edge. One complete circle would be considered one complete parallax scanning pattern. The display device would then be coordinated to display each corresponding complete circle of images at the rate of 3 to 6 Hz. The frame rate of the camera would dictate the number of complete images shown per view cycle.
While the single camera autostereoscopic imaging systems disclosed in the above cited prior art are effective in producing high quality, stable images that can be perceived in three-dimension when viewed on a conventional display device with the unaided eye, unfortunately these systems employ recording apparatus which are rather bulky and heavy, relatively complex in construction, and consume a meaningful amount of power in operation.